1. Field of the Invention
The present invention relates to a tunable dispersion compensator and a method for tunable dispersion compensation for compensating dispersion of an optical signal.
2. Description of Related Art
With the growing demand for information communication, there is a demand for greater transmission capacities. To meet this demand, efforts are being made to increase a transmission rate in an optical communication system. As the transmission rate is increased, a light waveform deteriorates due to chromatic dispersion in an optical fiber. The chromatic dispersion refers to a phenomenon that the rate at which an optical signal propagates through the optical fiber varies depending on the wavelength. Thus, a dispersion compensating fiber (DCF) is widely used to suppress the deterioration of the light waveform due to chromatic dispersion.
The DCF is designed to have a core diameter smaller than that of an ordinary optical fiber. This makes chromatic dispersion negative. This causes the chromatic dispersion of the DCF to have a characteristic having directionality opposite to that of the ordinary optical fiber. Therefore, combining the ordinary optical fiber with this DCF makes it possible to cancel chromatic dispersion produced by the ordinary optical fiber with the DCF and to suppress the deterioration of the light waveform.
However, the DCF does not have so great an amount of negative dispersion per length. Thus, the length of the DCF required to obtain an amount of dispersion compensation capable of long-distance communication is several tens of km. This causes a problem that the size of the equipment carrying out compensation of chromatic dispersion increases. Furthermore, since the DCF has quite a large transmission loss of an optical signal, an expensive light amplifier is required to compensate for the transmission loss. Furthermore, since the DCF has a small core diameter, the optical signal is trapped in a narrow area. This causes nonlinear interaction, deteriorating the light waveform. There is another problem that the amount of chromatic dispersion cannot be made variable. Thus, the use of the DCF involves many problems. Therefore, there is a demand for realization of a tunable dispersion compensator, which solves these problems.
A first proposal for this is a tunable dispersion compensator using a ring resonator formed of a PLC (planar lightwave circuit) (see Japanese Patent Application Laid-Open No. 06-224860, for example). Here, the PLC refers to a silica glass optical waveguide formed by depositing silica glass, etc., on a silicon or silica substrate. The PLC also includes an optical component or optical circuit with an optical fiber array connected to the silica glass optical waveguide. This first proposal is constructed of a one-stage ring resonator placed between optical fibers or between an optical fiber and optical multiplexed receiving apparatus, wherein an optical multiplexed signal is output with a time delay. This first proposal is an attempt to compensate for all wavelengths to be used at once for chromatic dispersion which the optical multiplexed signal receives inside the optical fiber.
However, since this first proposal uses only one stage of the ring resonator, the problem is that the range of wavelengths, dispersion of which can be compensated, is narrow. This involves another problem that when the wavelength of the optical signal used thereby increases and if the wavelength band used is broadened, chromatic dispersion cannot be compensated.
Thus, a second proposal is a structure using a plurality of ring resonators. FIG. 1 shows a tunable dispersion compensator with a plurality of ring resonators connected in series. This tunable dispersion compensator 101 is constructed of a first to nth ring resonators 1021, 1022, . . . , 102n, a waveguide 1031 which inputs an optical signal to the first ring resonators 1021, waveguides 1032, . . . , 103n which connect the first to nth ring resonators 1021, 1022, . . . , 102n in series and a waveguide 103n+1 which outputs an optical signal from the nth ring resonator 102n. The first to nth ring resonators 1021, 1022, . . . , 102n have the same structure. Therefore, the first ring resonator 1021 will be explained as a representative.
The first ring resonator 1021 is provided with a first coupler 104 having two inputs and two outputs connected to one end of the waveguide 1031 and a second coupler 105 also having two inputs and two outputs connected to one end of the waveguide 1032. First and second arms 106, 107 in a semicircular shape are connected to the output side of the first coupler 104 and input side of the second coupler 105 crossing each other midway, constituting a Mach-Zender interferometer. Furthermore, one end of the waveguide 1031 and one end of a semicircular third arm 108 are connected to the input side of the first coupler 104. Furthermore, the other end of the third arm 108 and one end of the waveguide 1032 are connected to the output side of the second coupler 105. The second arm 107 and the third arm 108 are provided with first and second heaters 109, 110, respectively.
This first ring resonator 1021 performs tunable dispersion compensation by controlling the first and second heaters 109, 110. That is, this ring resonator controls a delay time difference in the first and second arms 106, 107 by controlling the temperature of the first heater 109 disposed on only one arm making up the Mach-Zender interferometer. Furthermore, this ring resonator controls a resonating central wavelength by controlling the temperature of the second heater 110.
In order for this type of tunable dispersion compensator to perform dispersion compensation, the number of stages n of the ring resonators required is generally “4” or so. A case where the number of stages n is “4” will be considered by way of example. The first to fourth ring resonators 1021 to 1024 can set their respective amounts of dispersion to different values by controlling power to be applied to their respective first heaters 109. Furthermore, these ring resonators 1021 to 1024 can set their respective resonating central wavelengths to different values by controlling power to be applied to their respective second heaters 110 likewise.
FIG. 2 shows a principle of dispersion compensation at the tunable dispersion compensator 101 with the serial connection shown in FIG. 1. In FIG. 2, the vertical axis represents a delay time τ of an optical signal and the horizontal axis represents a wavelength λ. A chromatic dispersion characteristic 121 of the optical fiber represented by a two-dot dashed line indicates a general chromatic dispersion characteristic of an optical fiber. Compensating for this chromatic dispersion characteristic requires a chromatic dispersion characteristic 122 inclined in a direction opposite to the chromatic dispersion characteristic 121 to be given. Therefore, as shown in FIG. 2, the chromatic dispersion characteristic 122 is formed by combining chromatic dispersion characteristics of the respective ring resonators 1021 to 1024. Thus, combining the chromatic dispersion characteristic 122 of reverse inclination with the chromatic dispersion characteristic 121 cancels out the chromatic dispersion of the optical signal.
However, the tunable dispersion compensator 101 of the second proposal shown in FIG. 1 requires heaters 109, 110 twice as many for each of the ring resonators 1021 to 102n. Therefore, the number of parts increases and the amount of power required also increases. Furthermore, the power applied to these parts must be controlled individually with precision. This requires special control by an information processing apparatus such as a DSP (digital signal processor) or personal computer. Furthermore, acquiring a desired amount of chromatic dispersion requires an amount of power of each heater to be determined experimentally. This setting requires a long time and reduces productivity of the tunable dispersion compensator 101.
Therefore, as a third proposal, an attempt to reduce the number of heaters is shown in FIG. 3 (see Japanese Patent Application Laid-Open No. 2000-151513, for example). The second proposal shown in FIG. 1 disposes the first heater 109 and thereby controls delay times of the respective ring resonators 1021 to 102n to different delay times as shown in FIG. 2. Instead of this, the tunable dispersion compensator 111 of this third proposal sets coupling efficiencies κ1 to κ3 of the respective ring resonators 1121 to 1123 to different values and thereby sets phases φ1 to φ3 of the respective ring resonators 1121 to 1123 to different values. In this way, the delay times of the respective ring resonators 1121 to 1123 are changed as shown in FIG. 2. Each of the ring resonators 1121 to 1123 is provided with one of the heaters 1151 to 1153 for controlling a resonating central wavelength.
Here, with the tunable dispersion compensator 111 according to the third proposal shown in FIG. 3, the number of heaters is reduced compared to the second proposal. However, since the design differs from one ring resonator to another, the problem is that these ring resonators differ in the design, manufacture, control and inspection, which makes the structure of the tunable dispersion compensator 111 more complicated.
In these conventional proposals, delay time spectra of the respective ring resonators are basically spaced uniformly as shown in FIG. 2 to obtain a slope with a desired dispersion characteristic.